1. Field of the Invention
The present invention relates to an image sensor unit and an image reading apparatus.
2. Description of the Related Art
Examples of known image reading apparatuses include a copying machine, an image scanner, and a facsimile. Contact image sensor (CIS) units that optically read image information of originals to convert the image information to electric signals are used in the image reading apparatuses. Unlike a line sensor using a conventional reduction optical system that uses an optical system to reduce and read an image, a multi-chip image sensor including a plurality of sensor chips mounted in a line on the same substrate is used in the contact image sensor unit.
In general, sensor chips mounted on an image sensor unit are divided into a plurality of groups, and image signals are concurrently read in the groups. Each sensor chip includes a signal output amplifier of an output stage that amplifies and outputs an image signal to a common output line connected outside of the chip. The current consumption of the signal output amplifier is high. To reduce the current consumption of the image sensor unit, ON/OFF of the signal output amplifier is controlled in each sensor chip according to the operation state. Specifically, power supply to the signal output amplifier is started when reading operation is started in the sensor chip, and the power supply to the signal output amplifier is cut off when the reading operation is finished.
For example, in a period (blank period) after the end of the reading operation by all sensor chips mounted on the image sensor unit, circuits related to image signal output including the signal output amplifiers are turned off in all sensor chips, and the current consumption in the period is low. When a start pulse for starting the operation of the next line is input, all sensor chips perform vertical transfer operation, and the current consumption of the image sensor unit increases according to the total number of sensor chips. When the reading of the image signals from the sensor chips is started, the signal output amplifiers of the sensor chips that read the image signals are turned on, and the current according to the number of sensor chips that read the image signals.
Although an attempt is made to reduce the current consumption of the image sensor unit by controlling ON/OFF of the signal output amplifiers of the sensor chips, this increases the variation in the load. For example, before and after the blank period after the end of the reading operation in the sensor chips, the sensor chips that have been reading the image signals change from an ON-state to an OFF-state. The current consumption decreases according to the number of sensor chips that change from the ON-state to the OFF-state, and the current consumption of the image sensor unit significantly varies. The number of sensor chips that operate at the same time increases with an increase in the output of the image sensor unit, and the variation in the current consumption of the image sensor unit is large. The variation in the current consumption of the image sensor unit leads to variation in the power supply voltage supplied to the sensor chips and the like, and this causes variation in the output.
Japanese Laid-open Patent Publication No. 2006-166349 discloses an image forming apparatus including a filter on a power line from a power part to an original reading part and to an image processing part, and the filter removes variation in the power supply voltage caused by current variation in an original reading system to reduce the effect on an image writing part. A specific configuration is disclosed, in which a capacitor is placed on a power line from a power part to an original reading part and to an image processing part, and the current variation that occurs in an original reading system is charged and discharged to and from the capacitor to reduce the current variation on the power part side to reduce the variation in the power supply voltage.